Previously known from publication U.S. Pat. No. 6,082,081, is a film delivery device for a wrapping machine for delivering a wrapping film around an article to be wrapped up. The film delivery device comprises a first assembly which can be connected to the wrapping machine. Further, the film delivery device comprises a second assembly which is pivoted at the upper end to the upper end of the first assembly to turn about a horizontal hinge axis between a closed position and an open position.
The film delivery device also comprises a prestretch device comprised of two prestretch rollers. It comprises a first prestretch roller which is arranged to the first assembly in substantially vertical direction and arranged to be rotated at a first peripheral speed. Further, the film delivery device comprises a second prestretch roller which is arranged to be rotated at a second peripheral speed which deviates from the first peripheral speed, the second prestretch roller being arranged to the second assembly.
Both assemblies further comprise deflecting rollers for guiding the path of the wrapping film.
When the second assembly is in the closed position, the second prestretch roller is spaced apart from the first prestretch roller so that it is parallel to the first prestretch roller, and the first prestretch roller engages in frictional contact with the first face of the wrapping film and the second prestretch roller engages in frictional contact with the second face of the wrapping film in order to provide a so-called S-threading. An S-shaped threading of the film is known to be advantageous because it provides a large friction surface on the surface of the prestretch rollers. Furthermore, the distance between the points where the film is released from the surface of the upstream prestretch roller and where it contacts the downstream prestretch roller may be short in S-threading.
In the open position, the second assembly is turned, relative to the first assembly, about the hinge axis in such manner that the second prestretch roller is at an angle to the first prestretch roller, so that a gap which opens and widens downwards is provided between the first prestretch roller and the second prestretch roller, in which gap the film can be placed from below when the film roll is being changed.
A problem with the film delivery device known from publication U.S. Pat. No. 6,082,081, which represents the closest prior art, is that it is quite difficult to thread the film into the gap from below, in particular when the new film roll is being changed close to the floor level as is the case in some rotating-arm-type wrapping machines. A further problem is that the axis of the second prestretch roller in the second opening assembly must be pivoted at the upper end, i.e. a hinge joint must be provided between two axes, which impairs the structure and increases production costs. When a high prestretching degree is desired for the film, strong forces are applied to the joint, and hence the joint between the axes has to be dimensioned to be large. Furthermore, said joint makes the structure complex, expensive and vulnerable to malfunction.
Further known from publication U.S. Pat. No. 7,178,317 B1 is a film delivery device in which both prestretch rollers are in the same assembly while the deflecting rollers are in the opening assembly. The first and the second assembly are pivoted to each other at the lower end about a horizontal axis. The film can be easily threaded into the gap that opens upwards, but the prestretch system is not conformable to S-threading, but instead to so-called W-threading in which the prestretch rollers contact the same face of the film. In such construction, the problem relates to the W-threading in which a smaller friction surface is provided on the surface of the prestretch rollers than with S-threading. Furthermore, the distance between the prestretch rollers becomes relatively large with W-threading, resulting in that the wrapping film becomes too narrow. The S-threading is therefore more advantageous. Furthermore, in the construction according to the above-mentioned U.S. Pat. No. 7,178,317, it is difficult to change the degree of prestretching.